Double-Checked Lock Singleton in C++11

Question

Is the following singleton implementation data-race free?

static std::atomic m_instance;
...

static Tp &
instance()
{
    if (!m_instance.lo         


        

Answer 1

I think this a great question and John Calsbeek has the correct answer.

However, just to be clear a lazy singleton is best implemented using the classic Meyers singleton. It has garanteed correct semantics in C++11.

§ 6.7.4

... If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization. ...

The Meyer's singleton is preferred in that the compiler can aggressively optimize the concurrent code. The compiler would be more restricted if it had to preserve the semantics of a std::mutex. Furthermore, the Meyer's singleton is 2 lines and virtually impossible to get wrong.

Here is a classic example of a Meyer's singleton. Simple, elegant, and broken in c++03. But simple, elegant, and powerful in c++11.

class Foo
{
public:
   static Foo& instance( void )
   {
      static Foo s_instance;
      return s_instance;
   }
};